Partitioning Base Transceiver Station (BTS) equipment into a radio equipment (RE) part and a radio equipment controller (REC) part allows physical separation between the RE and REC. Separation has certain advantages, for example it allows one REC to control multiple, distributed REs. Here, “RE” should be understood as broadly encompassing base station repeater transceiver nodes.
An RE is connected via an analog RF feeder to the antenna that forms the BTS signaling air-interface. The fidelity of the signals transmitted and received via the antenna is degraded by feeder losses and hence it is advantageous for the RE to be placed close to the antenna. Typically, there are many REs assigned to an REC. Such an arrangement allows signal processing resources in the REC to be shared among the attached REs.
Any loss in fidelity of the signal as it is exchanged between an RE and its controlling REC can be effectively eliminated by first converting the analog signal that exists at the antenna into a digital signal within the RE. Hence, the interface between the REC and RE ideally is digital. The digitized signal sampled at a minimum sample rate required to faithfully represent the signal is often called the baseband. Other essential functions of the REC/RE interface include synchronization, control and management. Synchronization transfers timing information typically resident in the REC. Control and management provides access-control, operation, administration and maintenance.
Synchronization over the REC/RE link represents a key function. A master clock is typically resident in the REC and this clock must be transferred to the RE, so that the RE provides a synchronous air-interface (at the antenna). The master clock is traceable to a common time base in order to coordinate with other BTSs in the network. Put simply, the ability to maintain common and precise radio timing between an REC and its REs, and across multiple RECs and their respective REs, depends on accurate synchronization of radio timing at all REs.
Response-time processing represents another key consideration. In particular, response-time processing imposes a minimum round-trip latency criterion on the REC/RE link. As a practical matter, this requirement implies that no baseband samples may experience a delay that would exceed the ½ the minimum round-trip latency criterion.
An industry group has developed a standardized protocol for the REC/RE interface called Common Public Radio Interface (CPRI). CPRI is partitioned into the following data flows: control plane, management plane, synchronization plane and user plane. The user plane transfers the baseband; other flows are overhead. Comprehensive details regarding the CPRI specification are available in the interface specification document entitled, Common Public Radio Interface (CPRI); Interface Specification, V4.2 (2010-09-29).
The CPRI specification provides for the sort of precision and deterministic timing needed to transfer downlink data, timing, and control information from an REC to an RE and, conversely, transfer uplink data and control information from the RE to the REC. CPRI also provides for control and timing between an REC and two or more daisy-chained REs. Yet the CPRI protocol must be understood as a dedicated link; it is specialized for use in linking network nodes in the wireless communication network environment and it is not particularly robust with respect to transport impairments on the point-to-point links. In general, the specification assumes the use of synchronous, dedicated communication links between RECs and REs.
Such links may be impractical to implement, or they may not be economical as compared to other types of links. Asynchronous communication links, which may have multiple routing hops, may be cheaper and/or more easily deployed. As an example, Ethernet-based packet data networks are ubiquitous, and hence would be potentially advantageous for use as the REC/RE transport links. Yet, because of their asynchronous nature, such links are not particularly well suited for distributing the type of precisely synchronized data and control information that flows between RECs and REs.
There are known approaches for distributing timing synchronization information across asynchronous communication links. The IEEE 1588 standard represents one such approach. In particular, the IEEE 1588 standard proposes packet-based synchronization methods that can meet the stringent accuracy and reliability requirements of RE/REC synchronization. Packet-based synchronization as proposed by IEEE 1588 has two key attributes: (1) there is no dependency on the transport physical layer to provide inherent synchronization; and (2) the phase and frequency of the reference clock can be recovered.
Significant challenges remain with respect to using Ethernet or other asynchronous communication links, particularly with respect to the multiple hop scenario where asynchronous packets or frames transit through more than one router on the path between the routing endpoints. For example, because of the asynchronous nature of the Ethernet protocol, packets on an Ethernet link are subject to a variable queuing delay at every switching node on the routing path. Moreover, the basic Ethernet protocol does not guarantee that packets will be delivered in order, or even at all—although there are protocol mechanisms available for resending dropped packets. Thus, the transport link delay of the typical asynchronous communication link is variable rather than deterministic, which is not acceptable for the precise, deterministic timing required for CPRI-based synchronization between an REC and an RE.